These are both participles of the verb *auswählen*. German has two participles which have a slightly different function and luckily can be told apart very easily.

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The past participle has the marker **»ge«** at the beginning or before the main part of a separable verb. You will stumble across it very early when learning German as you need it for building the **Perfekt** tenses:

> Prof. Müller wählt Tests aus. *(Präsens, separable verb)*

> Prof. Müller *hat* Tests *aus**ge**wählt*. *(Perfekt)*

It's also used for **passive voice**:

> Die Tests *werden* von Prof. Müller *aus**ge**wählt*.

Your example uses the participle in place of an **adjective**.

> Prof. Müller zeigt mithilfe von aus**ge**wählten Tests etwas.

An *aus**ge**wählter Test* is a *chosen test*. The test has been chosen previously —that's the logic why German uses the past participle here, both past and passive— so it became chosen, the same way it could have become hard or simple (both real adjectives).

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The present participle has the marker **»d«** at the end. It has two uses. The first one is telling the main action happens at the same time as the action of the participle:

> Die Tests *auswählen**d*** beendete Prof. Müller seine Vorlesung.

*Choosing the tests, Prof. Müller ended his lecture.*

The other use is in your example, the participle in place of an **adjective**.

> Prof. Müller zeigt mithilfe von auswählen**d**en Tests etwas.

This second use is a bit tricky: The present participle is active voice, so now it's the test who do something! *Auswählen**d**e Tests* are tests that do the choice – between good and bad students.